When petroleum is found in a subterranean reservoir or formation, current field practice comprises penetrating the formation with the well, providing a flow conduit in the well and establishing fluid communication between the inside of the flow conduit and the petroleum-containing formation by perforation or other means, in order to permit the petroleum to flow or be pumped to the surface of the earth. Many subterranean, petroleum-containing reservoirs are comprised of a plurality of strata or separate earth formations, and it is not unusual for a producing well to be in contact with one or more separate intervals from which petroleum is recovered and also with one or more intervals from which water is recovered from the reservoir. Even in the early stages of primary recovery, some water production is not uncommon due to naturally occurring earth strata which have water present in the pore spaces thereof. Many formations overlie and contact an active aquifer, and production of large volumes of water from an underlying aquifer is a common result in such instances. Even if care is taken in completing the well only in the oil-saturated portion of the reservoir the greater mobility of water frequently results in substantial water production.
Numerous techniques have been disclosed and applied in field operations, for the purpose of reducing the permeability of water-producing intervals in a selective fashion, so as to reduce the flow of water without substantially reducing the flow of the desired petroleum fluids from the reservoir. One practice employed heretofore has been to inject liquid cement into the permeable formation and then to permit the cement to set, thereby rendering the formation so treated substantially fluid impermeable. Another method practiced has been to inject two treating fluids, one followed by the other, the fluids being selected on the basis that one or more components of one fluid will react with one or more components of the other fluid, resulting in the deposition of solid materials in the flow channels of the formation so treated, thus reducing the permeability of the treated portions of the formation.
Each of the foregoing discussed methods of plugging or reducing the porosity and/or permeability of an underground formation has some disadvantages. It is difficult to inject liquid cement into relatively tight formations such as are frequently encountered in petroleum production operations. Furthermore, it is difficult to control plugging with cement, and it is frequently found that the permeability of the oil bearing zone is reduced as much as the permeability of the water bearing zone, and thus production of both water and oil is made more difficult from a well thus treated. The use of two reactive fluids to form a plug in situ in a formation has not been entirely satisfactory due to a skin plugging effect at the interface between the two solutions as well as between the fluids and the formation. It has also been found difficult to obtain adequate mixing between the two separately-injected fluids in the formation so as to achieve effective reaction of the reactive component of fluids thus injected.
Treatment of subterranean formations with aqueous fluids, usually water or brine containing viscosity-increasing hydrophilic polymers such as polyacrylamide or polysaccharide, particularly in combination with injection of slugs of compounds which cause cross-linking between the polymers contained in the previously injected fluids and encourage the adsorption or attachment of polymers to the mineral surfaces of the earth formation have been developed. These have proven somewhat effective in certain applications but also suffer from serious operating problems. The polymers are not selective with respect to oil-containing and water-containing formations, and are as likely to reduce the permeability of the strata from which petroleum production is being taken as they are to reduce the permeability of strata from which water production is being obtained. Furthermore, the commonly available hydrophilic polymers are not stable with respect to temperature, and so these materials are not at all suitable for use in formations which are being stimulated by thermal means, such as by steam injection. Since thermal stimulation techniques have become a very popular method of increasing the production of oil from subterranean formations, particularly viscous, low API gravity oil, it is particularly desireable to provide a method for treating subterranean formations so as to reduce the permeability of water saturated intervals selectively, while not affecting the permeability of petroleum containing intervals, by application of a method which is suitable for use in high temperature environments such as those encountered in steam flooding or other thermal recovery means.
It is an object of the present invention to provide a formation treating method whereby the permeability of intervals in a formation penetrated by a well which are producing water can be reduced substantially while not reducing the permeability of the oil producing intervals to the same degree. It is another object of this invention to provide a thermally stable treatment for selective permeability adjustment. At least certain portions of these objects will be achieved by at least one embodiment of the present invention.